Quoting "congressional and law enforcement sources," CNN is reporting that an envelope sent to a senator's office has tested positive for the poison ricin.

"After the envelope tested positive in a first routine test, it was retested two more times, each time coming up positive, the law enforcement source said," CNN reports. "The package was then sent to a Maryland lab for further testing."

Quoting "two sources," Politico reports the envelope was sent to the office of Sen. Roger Wicker, a Republican from Mississippi.

NPR's Tamara Keith reports that Senate Majority Leader Harry Reid, D-Nev., said the package was sent to Wicker's office and it contained ricin or some other poison.

This is breaking news story. We'll update as we get more.

Update at 10:15 p.m. ET. More Testing

An FBI spokesman tells The Associated Press that initial field tests on the substance produced mixed results. It's now in the process of undergoing further testing at an accredited lab.

Update at 8:12 p.m. ET. Mail Facility Will Be Closed:

The Senate mail facility that received the letter with ricin will be closed for two or three days to complete an investigation, Reuters reports, citing the Senate's "chief security office."

Reuters adds:

The Senate's chief security office did not disclose the [sender's] name but said: 'The exterior marking on the envelope in this case were not outwardly suspicious, but it was postmarked from Memphis, Tennessee.'

"The FBI, the Capitol Police and other agencies are involved in the investigation, the Senate Sergeant at Arms said in an email to Senate employees."

Update at 7:15 p.m. ET. Reminders Of Anthrax Incidents:

Dick Durbin, a Democratic Senator from Illinois, says the letter, reminds him of the Sept. 11 anthrax incidents.

"We were told it's not as deadly or threatening but still being taken very seriously," Durbin said.

NPR's Tamara Keith tells us authorities believe this is an isolated incident.

Durbin said no other letters were found.

Update at 6:41 p.m. ET. Found At Processing Facility:

NPR's Tamara Keith reports:

"Sen. Mary Landrieu, a Democrat from Louisiana, says they were in a classified briefing about Boston when they came in and told them about the letter. It was found at an off site mail processing facility and no one in the Capitol complex is in danger."

Landrieu said the poison in question was ricin.

Update at 6:35 p.m. ET. What Is Ricin?

"Ricin is a poison found naturally in castor beans," the CDC writes in its fact sheet. "If castor beans are chewed and swallowed, the released ricin can cause injury. Ricin can be made from the waste material left over from processing castor beans."